1. Field of the Invention
The present invention relates to a method of manufacturing single-crystal silicon by the Czochralski pulling method (hereinafter called the CZ method).
2. Description of the Background
Single-crystal silicon is an essential element in the manufacture of various semiconductors and highly effective solar batteries and can be obtained industrially by the CZ method, wherein the single-crystal silicon is pulled up from a silicon melt. Another known method of manufacture is the floating zone method (the FZ method) in which a polycrystalline silicon rod is inserted into an induction coil to create single-crystal silicon. The CZ method is regarded as economical, because it requires less processing and because it provides a more profitable yield than the FZ method. However, the CZ method does not achieve single-crystal silicon of a quality equivalent to that obtained by the FZ method, mainly because of the fact that the resultant single-crystal silicon is contaminated by impurities from the worn quartz crucible containing the silicon melt.
Also, the continuous pulling of a single-crystal silicon rod for hours is a matter of course in the CZ method, since the continuous pulling may allow the single-crystal silicon rod to have a constant specific resistance in principle, thereby largely improving yield and achieving greater efficiency of furnace operation. However, no fully satisfactory continuous pulling operation is known in achieving these objectives primarily because of wear of the quartz crucible similar to deterioration of the quality of single-crystal silicon.
The introduction of an electromagnetic melting step into the CZ method has been recently watched with keen interest in light of the above objectives. The electromagnetic melting process is a kind of floating melting method, which uses a circumferentially divided water-cooled conductive crucible for melting a material placed therein as it floats through an electromagnetic field. The principle has been adopted in a polycrystalline silicon manufacturing apparatus as disclosed in Japanese Unexamined Patent Application Nos. 52962/1986, 264920/1989 and 30698/1990 and the like. The electromagnetic melting process may be employed in the CZ method thereby allowing a silicon melt placed in a crucible to be kept away from the inner surfaces of the crucible, in order to substantially reduce wear of the crucible in principle, thereby preventing the single-crystal silicon from being polluted by impurities from the crucible which ensures that the resultant single-crystal silicon has a quality equivalent to that obtained by the FZ method and which enables continuous pulling of single-crystal silicon for hours.
The above technique, however, has many problems and has not yet been practically used on an industrial basis. One of the factors hindering industrialization of the CZ method using the electromagnetic process is an agitation phenomenon of the silicon melt which is in the crucible, as the melt is kept away from the inner surface of the crucible. That is, when the induction frequency is high in the electromagnetic silicon melting process, current efficiency increases while the skin effect is also largely developed, so that eddy currents do not reach the central portion of the silicon placed in the crucible which results in an insufficient melting of the silicon in the central portion of the silicon melt. Hence, in order to make an enlarged single-crystal silicon rod, presently a lower induction frequency band width of practically 2 to 3 KHz is required. When the lower frequency band width is selected, the current acts deeply through the surface of the melted silicon to generate an electromagnetic force through cooperation of the current and a magnetic field, thereby causing a silicon melt which is placed in the crucible to be subjected to an intensive agitation force. The silicon melt which is kept in a floating state in the crucible is strongly agitated and forms waves on its liquid surface, thereby causing a single-crystal silicon rod pulled up from the silicon melt to have a swirl defect, dislocation and the like because of temperature fluctuations. In addition, the CZ method using the conventional electromagnetic melting process consumes almost half of the applied electric power to heat the crucible, so that it is not a very economical technique as expected in spite of the fact that silicon is heated directly by eddy currents. A need therefore continues to exist for improvements in the CZ method of forming single crystal silicon.